This invention relates to modem apparatus for estimating when the end of a transmitted message has been received.
In modulated carrier communication systems, each message is typically transmitted by modulating a carrier in accordance with an alphabet of discrete signal points (e.g., signal points which specify the amplitude and phase of the carrier) corresponding to the symbols in the message. In typical multipoint modem networks, i.e. those having a master modem receiver and a number of slave modem transmitters, the master modem polls the slaves successively to receive messages (each message usually being brief). Efficient operation requires the master to spend the minimum possible overhead time (in excess of the duration of the message itself) in communication with a given slave.
Masters therefore typically include detectors for determining when the end of a received message has occurred so that the master can promptly proceed to poll other slaves. Such detectors ordinarily sense the energy level of samples of the received modulated carrier (before equalization or demodulation) and signal the end-of-message when either the carrier energy level falls below a predetermined percentage of the past average carrier level, or the average carrier energy level (taken over a period longer than the baud interval) falls below a predetermined threshold.
Because transmission channel interference usually causes the modulated carrier to acquire a "tail" following each message (i.e., a period after the last message symbol during which the amplitude of the carrier slowly trails off, rather than dropping sharply, to zero), it is difficult to determine precisely when a message has ended. To avoid chopping off the end of the message, some detectors delay for a predetermined period after the carrier energy level has dropped below the threshold before signaling the end of message. In such detectors, a portion of a message may be chopped off if the predetermined delay is too short in a given case. To avoid such chopping, the predetermined delay is typically selected conservatively long, which often wastes time.